Cement production accounts for 5-6% of all anthropogenic carbon dioxide emissions. Society consumes a large amount of cement, and the cement industry releases approximately 9 kilograms (kg) of carbon dioxide for each 10 kg of cement produced. An alternative to this carbon dioxide intensive process is needed. The majority of the carbon dioxide emissions occurs during the decarbonation of calcium carbonate, CaCO3, to lime, CaO, while ˜40% of these emissions are from burning fossil fuels, such as coal, to heat the kiln reactors to ˜900° C.:CaCO3+Qheat→CaO+CO2  (1)nC+nO2→nCO2+Qheat  (2)
Calcium metal has a melting point of 842° C. CaCO3, as aragonite, decomposes at 825° C., and as calcite melts at 1339° C. In the 19th century, CaO was used to generate intense light, due to its high thermal stability (mp 2572° C.), giving rise to the phrase of being in the limelight. CaO is pervasively used by society. As with cement production, massive CO2 emissions are associated with the CaO formed for iron and aluminum production. Lime combines with silicate, phosphorous or sulfur impurities in metal ores. The products, such as calcium silicate slags, are immiscible, and can be removed from the molten metal. CaO heated with SiO2 and Na2CO3 forms glass. Lime for agriculture also contributes to carbon dioxide emissions. Lime, on reaction with solid carbon or water or is used in the synthesis of calcium carbide, CaC2, and acetylene, C2H2. Lime has widespread use in agriculture, and on reaction with solid carbon or water or is used in the synthesis of calcium carbide, CaC2(solid), and acetylene, C2H2(gas). CaO is used to make plastics opaque, and to create an alkaline pulp environment for paper production, and in refining sugar. Lime is used to scrub SO2 emissions from smoke stacks as calcium sulfite, CaSiO3, and in water treatment to soften water or remove phosphates from sewerage by precipitation of calcium phosphate, Ca3(PO4)2.
In forming CaO, solar thermal reactors have been studied used to replace the fossil fuel heat in eq. 2. However, solar thermal chemical reactions can be inefficient, and the majority of the carbon dioxide emissions still occurs (as decarbonation in eq. 1). In 2009 we introduced the theory of an efficient solar chemical process, based on a synergy of solar thermal and endothermic electrolysis. Solar heat, and the high concentration of molten reactants, substantially decreases the voltage needed to drive the electrolysis. Experimentally, the Solar Thermal Electrochemical Production (STEP) of energetic molecules can synthesize chemicals at solar efficiencies of 50%, hand as been demonstrated with the carbon dioxide free production of metals, fuels, bleach (chlorine) and for carbon capture.
There is a need for a new process for forming CaO without the release of carbon dioxide into the atmosphere.